1. Field of the Invention
The present invention relates to a technical field of a multilayer flexible wiring board, and particularly to a multilayer flexible wiring board suited for mounting semiconductor elements.
2. Detailed Description of the Related Art
Recently, there is a demand to decrease semiconductor devices in size and a multilayer flexible wiring board which enables to mount bare chip semiconductor elements is regarded as important.
Now, referring to FIG. 8(a), reference numerals 110, 130 denote two kinds of flexible wiring board piece having a single layer structure. Fabricating steps of a multilayer flexible wiring board according to a conventional art will be explained by use of these flexible wiring board pieces 110, 130.
Out of the two kinds of flexible wiring board pieces 110, 130, one flexible wiring board piece 110 comprises a base film 112, a metal wiring 115 arranged on the base film 112, and a cover film 117 adhered on the metal wiring 115.
The metal wiring 115 and cover film 117 are patterned in predetermined shapes, respectively. Thereby the cover film 117 has a plurality of openings 119 formed therein. Each of the openings 119 is positioned above the metal wiring 115.
The other flexible wiring board piece 130 comprises a base film 137, a metal wiring 135 arranged on the base film 137, and a cover film 132 adhered on the metal wiring 135.
The base film 137, cover film 132 and metal wiring 135 are patterned in predetermined shapes, respectively. Thereby the cover film 132 and base film 137 have a plurality of openings 138, 139 formed therein, respectively.
Each of the openings 138, 139 is positioned above the metal wiring 135. At the bottom of the opening 139 provided in the base film 137, the surface of the metal wiring 135 is exposed. On the other hand, in the opening 138 provided in the cover film 132, copper is filled by metal plating method. The copper is deposited to protrude from the cover film 132 surface and a solder coating 140 is formed at the tip thereof by metal plating method. As a result, a metal projection 142 is composed of a projection body 136 and the solder coating 140.
To fabricate a multilayer flexible wiring board by adhering the above-mentioned flexible wiring board pieces 110, 130 to each other, first, as shown in FIG. 8(a), two flexible wiring board pieces 110, 130 are positioned to face the cover films 117, 132 thereof and a thermoplastic resin film 151 is interposed therebetween to close in contact with each other.
Upon heating, the thermoplastic resin film 151 is softened so as that an adhesive force thereof appears. Accordingly, when the flexible wiring board pieces 130, 110 are pressed and heated to soften the thermoplastic resin film 151, the cover films 117, 132 are adhered to each other. At the same time, the metal projection 142 is forced in the softened thermoplastic resin film 151, thereby the tip of metal projection 142 comes in contact on the metal wiring 115 (refer to FIG. 8(b)).
In this state, a soldering metal composing the solder coating 140 is melted when the temperature of the flexible wiring board pieces 110, 130 further increases. After cooling, the projection body 136 and the metal wiring 115 are connected with each other by the solidified solder metal 155 to obtain a multilayer flexible wiring board 150 (refer to FIG. 8(c)).
When an electric part such as a semiconductor chip is connected with the flexible wiring board 150, first they are appropriately positioned to each other. Next, as shown in FIG. 9(a), a bump 175 formed on an element body 171 of an electric part 170 is positioned above the metal wiring 135 exposed at the bottom of the opening 139 in the base film 137, and the tip of the bump 175 is allowed to come in contact onto the surface of the metal wiring 135.
The bump 175 is composed of a projection body 172 including copper and a solder coating 173 formed thereon. When the electric part 170 is heated while the tip of the bump 175 is contacted with the metal wiring 135, the solder coating 173 is melted to connect the projection body 172 with the metal wiring 135.
Thus, the electric part 170 is connected with multilayer flexible wiring board 150 via solder coating 173.
In the above-mentioned flexible wiring board 150, however, the solder metal 155 within the flexible wiring board 150 is also heated when the solder coating 173 of the semiconductor chip 170 is melted.
In this case, connections deteriorate when the solder metal 155 becomes brittle or the solder metal 155 is re-melted. Reference numeral 156 in FIG. 9(b) denotes a connecting portion between the metal wirings 115, 135 within the flexible wiring board 150. It shows that the solder metal 155 is re-melted and the connection between the metal wirings 115, 135 worsens.
An object of the present invention is to provide a multilayer flexible wiring board in which the above-mentioned inconvenience does not occur and also to provide a fabricating method thereof.
In order to solve the above-mentioned problems, the present invention provides a fabricating method of a multilayer flexible wiring board, said method comprising the step of laminating a flexible wiring board piece having a metal wiring, in which a metal coating is exposed on at least a part of surface of said metal wiring, to another flexible wiring board piece having metal projections on the surface of which metal coating is formed, wherein one of or both of the surface of said metal coating formed on said metal wiring and on the surface of said metal projections is composed of a soft metal coating having a Vickers"" hardness of 80 kgf/mm2 or lower, and wherein the method further comprises the step of applying ultrasonic wave thereto in a state where said metal coating formed on said metal wiring contacts with the metal coating formed on the surface of said metal projections to connect said metal wiring with said metal projections.
Furthermore, the present invention provides a fabricating method of a multilayer flexible wiring board, said method comprising the step of adhering a flexible wiring board piece having a metal wiring, in which metal coating is exposed on at least a part of surface of said metal wiring, to another flexible wiring board piece having metal projections on the surface which metal coating is formed, wherein one of the metal coating formed on said metal wiring and the metal coating formed on the surface of said metal projections is composed of soft metal coating having a Vickers"" hardness of 80 kgf/mm2 or lower, the other one of the metal coating formed on said metal wiring and the metal coating formed on the surface of the metal projections is composed of hard metal coating having a Vickers"" hardness of 120 kgf/mm2 or higher and soft metal coating having a Vickers"" hardness of 80 kgf/mm2 or lower laminated on the said hard metal coating, and wherein the method further comprises the step of applying ultrasonic wave thereto in a state where the soft metal coating formed on said metal wiring contacts with the soft metal coating formed on said metal projections to connect said metal wiring with the metal projections.
Still further, the present invention provides the fabricating method of a multilayer flexible wiring board of the foregoing, wherein a thermoplastic film is interposed between said flexible wiring board pieces, adhering said flexible wiring board pieces to each other by said thermoplastic film in advance before applying ultrasonic wave thereto, and pressing at least one of said flexible wiring board pieces to contact the metal coatings with each other while applying ultrasonic wave.
Moreover, the present invention provides the fabricating method of a multilayer flexible wiring board in the foregoing, wherein at least one of said flexible wiring board pieces is heated to 50xc2x0 C. or higher when applying ultrasonic wave thereto.
The present invention also provides a multilayer flexible wiring board in which at least two layers of metal wirings are laminated while interposing at least an insulating layer therebetween, a metal projections provided on one of said metal wirings are connected with the other metal wiring by the application of ultrasonic wave, wherein soft metal coating having a Vickers"" hardness of 80 kgf/mm2 or lower is formed on at least one of surfaces of said metal projections and said metal wiring to be connected with said metal projections.
In the foregoing, the present invention further provides the flexible wiring board wherein said soft metal coating is composed of one or more metal coatings selected from the group consisting of a gold coating including gold as a main component, a platinum coating including platinum as a main component, a silver coating including silver as a main component, and a palladium coating including palladium as a main component.
Furthermore, the present invention also provides a multilayer flexible wiring board in which at least two layers of metal wirings are laminated while interposing at least an insulating layer therebetween, metal projections provided on one of said metal wirings are connected with the other metal wiring by the application of ultrasonic wave, wherein soft metal coating having a Vickers"" hardness of 80 kgf/mm2 or lower is formed on one of surfaces of said metal projections and the metal wiring connected with said metal projections, and hard metal coating having a Vickers"" hardness of 120 kgf/mm2 or higher and soft metal coating having a Vickers"" hardness of 80 kgf/mm2 or lower are layered on the other of the surfaces of said metal projections and the metal wiring connected with said metal projections, and wherein the soft metal coating formed on the surface of said metal projections are connected with the soft metal coating formed on said metal wiring by the application of ultrasonic wave.
The present invention provides the flexible wiring board in the foregoing, wherein said hard metal coating is composed of nickel coating including nickel as a main component.
The present invention provides the flexible wiring board in the foregoing, wherein one of or both of said soft metal coatings on surfaces of said metal projection and said metal wiring is composed of one or more metal coatings selected from the group consisting of a gold coating including gold as a main component, a platinum coating including platinum as a main component, a silver coating including silver as a main component, and a palladium coating including palladium as a main component.
The present invention also provides an electric device comprising the flexible wiring board and an electric part connected with said flexible wiring board by soldering.
Since the present invention is composed as described above, the metal coating formed on the surface of metal projections contact with the metal coating formed on the metal wiring and they are connected with each other by the application of ultrasonic wave. Accordingly, the metal wirings of the flexible wiring board piece are not connected by soldering, but they are connected by metal bonding between the metal coating. Even if the solder of the electric part is heated to be melted when the electric part is connected with the multilayer flexible wiring board by soldering, the disconnection between the metal wirings and the deterioration of the connection therebetween can be avoided.
At least one of the metal coating contacting with each other is composed of a soft metal coating having a Vickers"" hardness of 80 kgf/mm2 or lower (where 1 kgf/mm2 equals to 9.80665 N/mm2) Accordingly, soft metal coating is bonded to the other metal coating by the application of ultrasonic wave reliably, thereby enhancing the reliability of the multilayer flexible wiring board of present invention.
The metal coating connected with the soft metal coating may be a soft metal coating or may be a hard metal coating a surface of which has a Vickers"" hardness of 120 kgf/mm2 or higher. Moreover, the hard metal coating and the soft metal coating are laminated with each other.
In particular, if a soft metal coating composed of a gold coating is contacted with a soft metal coating composed of a gold coating to connect them by the application of ultrasonic wave, a multilayer flexible wiring board having a high thermal fatigue resistance can be obtained because of its connectability between the gold coatings.
In the case where a hard metal coating including a nickel coating is formed as an under layer for the gold coating, ultrasonic vibration is strongly concentrated at a contacting portion between the gold coatings, thereby enhancing the connection.
Furthermore, in the case where flexible wiring board pieces are adhered to each other by use of an adhesive layer such as a thermoplastic resin film before the application of ultrasonic wave, positional shift on connecting position of metal projection and metal wiring can be prevented during the application of ultrasonic wave.
The flexible wiring board piece and the multilayer flexible wiring board of the present invention have property of flexibility.